1. Field of the Invention
This invention relates generally to the preparation of diaryl ethers or diaryl sulfides by a process involving a thoria-containing catalyst and more particularly concerns the use in such process of a catalyst comprising thoria deposited on neutral support particles.
2. Description of the Prior Art
The commercial importance of diphenyl ether-based monomers and diphenyl sulfide-based monomers in engineering polymer formulations is well recognized. Either such materials are themselves potential useful monomers or can serve as precursors to monomers of proven utility such as oxybisanaline. For example, 4,4'-diaminodiphenyl ether is currently used in the preparation of Torlon.RTM. and of Vespel.RTM.. Other diphenyl ether-based monomers which are of potential commercial importance include diacids, diols or dianhydrides of diphenyl ether. Although diphenyl ether-based monomers and diphenyl sulfide-based monomers have proven utility, their high cost and the lack of a convenient method of preparation have hindered any large volume applications of such monomers.
A potentially inexpensive method of preparing diphenyl ether-based monomers and diphenyl sulfide-based monomers is the catalyzed dehydrative coupling of phenols or thiophenols, respectively. Unsupported and supported thorias have been employed as catalysts in the dehydrative coupling of phenols or thiophenols to form the corresponding diphenyl ethers of diphenyl sulfides, respectively. For example, British Patent Specification No. 911,246 discloses a method for the dehydrative coupling of a phenol to the corresponding diphenyl oxide in the presence of a catalyst containing thoria supported on alpha alumina, preferably in the vapor phase, and at substantially atmospheric pressure and at a temperature between 300.degree. C. and 550.degree. C., for example, 475.degree. C. The catalyst was prepared by impregnating alumina with thorium nitrate tetrahydrate and then decomposing the thorium nitrate to form thorium oxide. The patent states that, under conditions conducive to high catalyst activity in the dehydrative coupling reaction, the activity of the catalyst disclosed therein decreased with use. Furthermore, the data in the patent illustrates that the coupling reaction did not take place stereospecifically.
Karuppannasamy et al., "Investigations of Phenol Decomposition on Thoria Catalysts," in Proc. Natl. Symp. Catal., 4th, 1978, pp. 443-450, and "Reactions of Phenols and Alcohols over Thoria," in Journal of Catalysts, Vol. 63, pp. 433-437 (1980), reported on studies of the dehydrative coupling of phenols in the presence of a thoria catalyst that was prepared by a procedure involving precipitation from a thorium nitrate solution by addition thereto of ammonia. The article contains no mention of a catalyst support and evidently the thoria catalyst was unsupported. The authors also indicated that thoria catalysts can also be prepared by the thermal decomposition of thorium nitrate and thorium oxalate. In the article in the Journal of Catalysts, the authors indicated that the results on catalysts prepared by these three different routes were comparable qualitatively.
Karuppannasamy et al., "Reactions of Phenols and Alcohols over Thoria: Mechanism of Ether Formation," Journal of Catalysts, Vol. 66, pp. 281-289 (1980) reported on the dehydrative coupling of phenols in the presence of a thoria catalyst that was prepared by a procedure involving the thermal decomposition of thorium oxalate. The article contains no mention of a catalyst support, and evidently the thoria catalyst was unsupported. The authors also indicated that thoria catalysts can also be prepared by the thermal decomposition of thorium nitrate or the calcination of thorium hydroxide and that the results on catalysts prepared by these three different routes were qualitatively similar.
Briner and Bron, "Catalytic Dehydration of Phenols; Influence and Nature of the Position of Substituted Groups," Heb. Chin. Acta., Vol. 15, pp. 1234-1241 (Aug. 1932) disclose the use in the aforesaid dehydrative coupling of phenols of a thoria catalyst. The method of preparation of the catalyst was not disclosed.
Sabatier et al., Computes Rendies, 1912, p. 260 and 1914, p. 608 disclose the use of unsupported thoria to catalyze the dehydrative coupling of phenols.
However, the selectivity of such thoria catalysts for the production of certain, highly desirable stereospecific substituted diaryl ethers and diaryl sulfides is often less than is generally acceptable for a commercial process. Furthermore, as disclosed in British Patent Specification No. 911,246, upon use under the conditions generally employed in the dehydrative coupling of phenols, thoria catalysts typically experience a significant loss of activity for such coupling reactions, with a further reduction in their selectivity for the production of certain, highly desirable stereospecific substituted diaryl ethers.